Acoustic attenuator providing relatively high insertion loss at low frequencies

ABSTRACT

A flowing fluid (e.g., air, gas, and water) through which acoustic pressure waves are propagating is passed through an attenuator comprising, in accordance with one embodiment of the invention, multiple parallel longitudinal fluid flow paths wherein the fluid flowing in each longitudinal path successively encounters a number of short, thin, flat plates which are arranged in series and longitudinally spaced along said path. The longitudinal spacing between each plate in series in each path is of sufficient length, as is the length of each plate, to ensure the formation of separate and distinct boundary layer flow regions on each of the plates. Low frequency acoustic pressure waves excite the boundary layer flow regions on the surfaces of each plate in the series so that the boundary layers become oscillatory and are shed from the trailing edges of each plate in the form of oscillatory wakes. The fluid flow character in the oscillatory boundary layers and in the oscillatory wakes is rotational and does not effectively propagate low frequency acoustic pressure waves into the far field. Accordingly, a fraction of the low frequency acoustic energy is repetitively attenuated at each plate in the series by the conversion into a non-acoustic form of energy. Intermediate and high frequency acoustic energies are also attenuated because of such effects as: scattering by the plates; viscous dissipation of the acoustic energies at the surfaces of the plates; and, axial acoustic impedance mismatching enhanced by the formation of separation bubbles at the leading edges of each of the plates.

4' United States Patent Savkar et al.

[451 Jul 22,1975

[ ACOUSTIC ATTENUATOR PROVIDING RELATIVELY HIGH INSERTION LOSS AT LOW FREQUENCIES [75] Inventors: Sudhir D. Savkar; Walter B. Giles;

Robert B. Tatge, all of Scotia, NY.

[73] Assignee: General Electric Company,

Schenectady, N.Y.

22 Filed: Mar. 4, 1974 21 Appl. No.: 447,953

Related US. Application Data [63] Continuation of Ser. No. 308,323, Nov. 21, 1972.

[56] References Cited UNITED sTATEs PATENTS 2,838,132 6/1958 Markham et al 181/33 0 ux 3,038,552 6/1962 Hedblom 181/63 x 3,111,190 11/1963 Vaughn 181/63 X 3,111,191 11/1963 Bachert 181/56 3,177,972 4/1965 Wirt 181/56 3,195,679 7/1965 Duda et a1. 181/33 HB UX 3,623,295 11/1971 Shriner 181/56 x FOREIGN PATENTS OR APPLICATIONS 636,676 5/1950 United Kingdom 181/46 663,241 12/1951 United Kingdom 181/46 37,279 4/1906 Switzerland 181/63 .Primary Examiner-Stephen' J. Tomsky Assistant Examiner.l0hn F. Gonzales Attorney, Agent, or Firm- -Stephen B. Salai; Joseph T. Cohen; Jerome C. Squillaro r [57] ABSTRACT A flowing fluid (e.g., air, gas, and water) through which acoustic pressure waves are propagating is passed through an attenuator comprising, in accor dance with one embodiment of the invention, multiple parallel longitudinal fluid flow paths wherein the fluid flowing in each longitudinal path successively encounters a number of short, thin, flat plates which are arranged in series and longitudinally spaced along said path. The longitudinal spacing between each plate in series in each path is of sufficient length, as is the length of each plate, to ensure the formation of separate and distinct boundary layer flow regions on each of the plates. Low frequency acoustic pressure waves excite the boundary layer flow regions on the surfaces of each plate in the series so that the boundary layers become oscillatory and are shed from the trailing edges of each platein the form of oscillatory wakes. The fluid flow character in the oscillatory boundary layers and in the oscillatory wakes is rotational and does not effectively propagate low frequency acoustic pressure waves into the far field. Accordingly, a fraction of the low frequency acoustic energy is repetitively attenuated at each plate in the series by the conversion into a non-acoustic form of energy. Intermediate and high frequency acoustic energies are also attenuated because of such effects as: scattering by the plates; viscousdissipation of the acoustic energies at the surfaces of the plates; and, axial acoustic impedance mismatching enhanced by the formation of separation bubbles at the leading edges of each of the plates.

25 Claims, 17 Drawing Figures PATENFEDJULZZ was I PATENTE DJmms SHEET J NI butmmvmkk QQQQ\ QQQ\ QQ\ Q\ Q QRQ QQ I qp-(Jamo p 5.907 (Jo/7.12671] PATENTEDJUL22 ms ShEET mmmmm mmmmm mmm mmm

mmzmmm mmmzmm mamuzcnmm mmnmm 7 contract and has reserved the'rights set forth in sections -tic filters of the muffler type. However, .for a number such absorbent materials loose their attenuation prop- ACOUSTIC ATTENUATOR PROVIDING RELATIVELY men INSERTION Loss AT LOW FREQUENCIES BACKGROUND OF THE INVENTION Nate rated by gas turbines, aircraft engines, etc. anti propagated as acot' 'stic pressure waves in the fluids flowing through such machines is botha nusiance and a hazard. Fora variety of reasons suppression of such noise at the source (i.e., in and at the machine) is not always possible or practicall'Hence, much effort has been directed toward suppressing such noise after it has been generated at locations relatively remote from the Set. No.

machine where the noise is being generated. Thus, a va- 5 riety of silencers for suppressing noise in fluid streams have been proposed. In an effort to keep fluid flow energy losses at minimum levels many of the proposed silencers have employed Helmholtz resonators or acousof reasons such silencers have not provensatisfactory, especially at low frequencies. For example, in order to provide adequate suppression orfiltering of low frequeasy noise such silencers require chambers having intolerable large dimensions. Moreover, in order to increase the attenuation quantities of so-called sound absorbent materials are used in prior art silencers. Forexample, mufflers are packed with fibrous materials which absorb some'of the acoustic energy. However,

erties with age; e.g., such materials are contaminated with water and debris as time passes;

SUMMARY OF THE INVENTION 7 sure waves, is particularly effective'in attenuating the acoustic energy content of low frequency acoustic pressure waves.

Another object of the inventionis to provide a broad--. band acoustic attenuator for'attenuating the acoustic energy content of low, intermediate and high frequency acoustic pressure waves which are being propagated in a moving fluid such as, for example, an airstream,"gaseous stream, etc.

Another object of the invention is to provide an acoustic attenuator which does not employ acoustic energy absorbing material, such as the fibrous absorbent material commonly used in inany prior art silencing devices. i

Another object of the invention is" to provide. an

' acoustic attenuator which is relatively compact, easily 0 constructed and rugged.

The subject acoustic attenuator is intended for insertion in the flow path of a noise-propagating moving fluid and in accordance with one illustrative embodiment of the invention there is provided an acoustic attenuator, for attenuating the acoustic energy content' of acoustic pressurewaves, embracing a broad band of acoustic frequencies, propagating through a flowing fluid, comprising a longitudinal conduit having inlet and outlet openings which are spaced apart from each other along a longitudinal central axis of the conduit. The inlet and outlet openings define an entrance and exit, respectively, for the flowing fluid aforesaid which the aforesaaid acoustic pressure waves are propagating. Situated within the conduit is a multiplicity of short axially-extending serially-arranged thin, flat plates; each plate being stationary andhaving a pair of parallel flat surfaces disposed in back-to-back relationship as well as a leading edge and a trailing edge. Each series of plates extends longitudinally from the inlet opening to the outlet opening of the conduit and the plates in each series are longitudinally separated from each other so that spaces exist between adjacently situated plates; i.e., a space between the trailing edge of one plate and the leading edge of the next plate. The fluid, through which the acoustic pressure waves are propagating, enters the inlet opening and flows in a longitudinal or axial direction over the back-to-back parallel flat surfaces of each plate in each series of plates and finally exits at the outlet opening of the attenuator. It has been discovered, and verified by testing, that there occurs a relatively high attenuation of the acoustic energy content of low frequency acoustic pressure waves being propagated through the flowing fluid as it passes through such an attenuator. It has also been discovered, and verified by testing, that the acoustic energy content of intermediate and high frequency acoustic pressure waves propagating through the fluid passing through'suchan attenuator is significantly attenuated.

Hereinafter set forth inconnection'with the detailed description of various embodiments of the subject attenuator are theoretical explanations for the aforesaid attenuations; e.g., a theoretical explanation of the. at-

tenuation occurring at low frequencies is extensively vantages of the inventionfappear hereinafter wherein the invention is disclosed by means of illustrative embodiments.

DRAWINGS FIG. 1 is a perspective view of a first embodiment of an acoustic attenuator according to the invention.

FIG. 2 is another perspective illustration showing the acoustic attenuator of FIG. I located in a fluid flow path between intake and'exhaust ducts through which noise-propagating fluid is flowing; the noise, or acoustic energy disturbances, being generated, initially, by a source such as, for example, a gas turbine.

FIG. 3 is an interrupted end view of the acoustic attenuator of FIG. 1 showing an entrance for flowing fluid; said flowing fluid having acoustic pressure waves of low, intermediate and high frequencies propagating therethrough.

FIG. 4 is an interrupted longitudinal cross section, taken alongthe section line 4-4 in FIG. 3, showing the interior of the acoustic attenuator of FIG. 1.

FIG. 5 is an enlarged fragmentary view, similar to that shown in FIG. 4, showing, inter alia, some fluid flow patterns at the plates of the acoustic attenuator of FIG. 1.

FIG. 5a is'an enlarged longitudinal section view of one of the plates of the attenuator of FIG. 1.

FIGS. 6, 7, 8 and 9 are diagrams for four successive instants of time showing: a velocity profile of the fluid flow in the boundary layer region adjacent the surface of one plate of the attenuator of FIG. I; a separate velocity profile of fluid particle movement in the same boundary layer flow region caused by low frequency acoustic pressure waves; and, a composite, or resultant,

fluid velocity profile obtained by the superposition of both of the aforesaid velocity profiles.

FIG. 10 is a graph showing the insertion loss, or acoustic attenuation, obtained for various mass flows of air passing through the acoustic attenuator of FIG. 1 and propagating acoustic pressure waves of a broad range of frequencies.

FIG. 11 is a graph showing the calculated insertion loss, or acoustic attenuation, provided by an acoustic attenuator like that shown in FIG. 1, for different flow parameters as a function of acoustic pressure wave frequency.

FIG. 12 is another graph showing the insertion loss, or acoustic attenuation, provided by an acoustic attenuator, like that shown in FIG. I, having rows of attenuator plates with different numbers of serially arranged plates as a function of acoustic pressure wave frequency.

FIG. 13 is a diagram of a second embodiment of the acoustic attenuator of the invention showing a staggered configuration of attenuator plates, similar to that shown at FIG. 4, but including some plates having thicker cross sections which are included in the subject attenuator for improving the insertion loss, or attenuation, of intermediate and high frequency acoustic energies.

FIG. 14 is a diagram of a third embodiment of the acoustic attenuator of the invention showing a regular row and column array of attenuator plates, all plates having the same thickness dimension.

FIG. 15 is a diagram of a fourth embodiment of an acoustic attenuator in accordance with the invention showing a row and column array of attenuator plates, like that shown in FIG. 14, but including some plates having thicker cross sections, said thicker plates being included for improving the insertion loss, or acoustic attenuation, of intermediate and high frequency acoustic energies.

FIG. 16 is a diagram of a fifth embodiment of an acoustic attenuator in accordance with the invention showing angularly arranged, or canted, attenuator plates, some of which have thicker cross sections than others; the angular disposition of the plates and the inclusion of plates having thicker cross sections being purposeful for improving the insertion loss, or acoustic attenuation, of intermediate and high frequency acoustic energies.

DETAILED DESCRIPTION An acoustic attenuator 20 in accordance with a first embodiment of the invention is illustrated in perspective views at FIGS. 1 and 2. As shown at FIG. 2 the attenuator 20 is located in a fluid flow path in series between an intake duct 22 and an exhaust duct 24. The intake duct 22 is coupled to a source 26 from which a fluid (e.g., air, gas or water) is discharged into the intake duct 22. The source 26 also generates noise; i.e., acoustic pressure waves of low, intermediate and high frequency. The acoustic pressure waves are propagating through the flowing fluid, i.e., leaving the source 26 and entering the intake duct 22. Source 26 may, for example, be a jet engine, gas turbine, etc. The source 26 may, at any given instant of time or during a given time period, be generating a particular acoustic frequency or band of acoustic frequencies. As shown in FIG. 2 the fluid flow directions are indicated by the labeled arrows. In the descriptions and discussions which appear hereinafter, it is to be assumed, unless otherwise stated, that the acoustic pressure waves generated by source 26 are propagating through the flowing fluid in the same general direction as the fluid flow direction; i.e., in the direction indicated by the labeled arrows showing fluid flow direction in FIG. 2.

In FIGS. 1, 2, 3 and 4 structural details of the acoustic attenuator 20 are illustrated. As shown, the attenuator 20 is comprised of four wall members 28 28 defining a conduit having inlet and outlet openings 30 and 32, respectively spaced from each other at opposite ends of a longitudinal central axis through the center of the conduit. The longitudinal central axis extends in the general direction of the fluid flow direction. Inlet opening 30 is, as shown in FIG. 1, divided into a number of inlet sections 34, 36, 38, 40, 42, 44, 46, 4s, 50 and 52.

These inlet sections 34 52 are separated from each other by wall members 54 54. The wall members 54 54 extend in parallel, longitudinally from inlet opening 30 to the outlet opening 32. Thus, each of the wall members 54 54 is also disposed in parallel relationship with two of the wall members 28 as indicated at FIG. I. For each of the inlet sections 34 52 there is a corresponding outlet section at the outlet opening 32 of the conduit forming the attenuator 20. Some of the aforesaid outlet sections are identified hereinafter in connection with FIGS. 3 and 4.

FIG. 3 is an end view of the acoustic attenuator 20 showing portions of the first and last inlet sections 34 and 52 of inlet opening 30.

FIG. 4 is a longitudinal cross section, taken along section line 4-4 of FIG. 3, of a portion of the acoustic attenuator 20. As shown in FIG. 4, 'an outlet section 56 of the outlet opening 32 is situated longitudinally opposite a corresponding inlet section 52 of inlet opening 30. For convenience of illustration, consistent with clarity, the views shown in FIGS. 3 and 4 are illustrated with a number of breaks in the height, width and length dimensions of the attenuator 20. Looking into the inlet section 52 (FIG. 3) one can observe the leading edges 34 50 comprising the inlet opening 30 also present the leading edges of a like number of thin flat plates. For example, the inlet section 52 includes a first row of plates 58A, 62A XXA, XZA. As indicated at FIGS. 3 and 4, beyond inlet opening 52 and the'first row of plates, hereinbefore identified, is a second row of plates 60A, 64A XYA. As shown in FIG. 4 the plates 60A XYA.'of the second row of plates are staggered, or offset, relative to the plates 58A XZA of the first row of plates, the plates of the second row dividing the spaces between, the plates of the first row substantially in half. Similarly, as indicated at FIG. 4 there is a third row of plates 58B, 62B XXB and XZB. Each of the plates in the third row is in line with a corresponding plate 58A XZA in the first row of plates. Thus, in the first, third Nth rows (odd numbered rows) the respective plates 58A, 58B 58N form a series of spaced-apart, thin, flat plates which extend longitudinally, in an axial direction, substantially parallel to the flow of fluid (i.e., within a few degrees), fromthe inlet section 52 of inlet opening 30 to the outlet section 56 of the outlet opening 32 of attenuator 20. Similarly, in the second, fourth Nth rows the respective plates 60A 60N form another series of spaced apart plates which extend longitudinally, in an axial direction, substantially parallel to the flow of fluid (i.e., within a few degrees) from inlet section 52 to outlet section 56 and the plates in these even numbered rows are staggered, or offset, relative to the plates in the odd numbered rows. All of the aforesaid plates 58A XZN may be suitably secured at one end thereof to the wall member 28 and to the other end thereof to the wall member 54. In the alternative, bent ribbonsof metal resembling corrugated strips may be employed (as suggested in FIG. 1) with the vertical sections of such metal ribbons serving as the individual plates. Thus, the plates 58A XZN are rigid and stationary relative to each other as well as relative to wall members 28 and 54.

FIG. 5 is a fragmentary view of part of FIG. 4 showing enlarged illustrations of some of the thin flat plates 58B, 60A, 60B, 62B, 64A, 64B and 66B. The aforesaid plates are disposed in the flow path between inlet sec-- tion 52 and outlet section 56 of the attenuator 20. As indicated each plate includes a pair of smooth, flat surfaces S1 and S2 arranged back-to-back. The cross section dimension of each plate is relatively thin as compared with its length. Each plate includes a leading edge LE as well as a trailing edge TE; the former being subjected to oncoming fluid flow before the latter. For example, in the enlarged view, FIG. 5a, the plate 60A includes: the pair of flat, smooth, back-to-back surfaces S1 and S2; the leading edge LE: and, the trailing edge TE. As shown in FIGS. 5 and 5a, fluid flowing in the direction indicated by the FLOW arrow initially contacts the leading edges LE of the variousplates. Separation bubbles, as indicated, are formed at each of the leading thereof. For example, at plate 60A in FIG. 5a, the flowing fluid forms separation bubbles at the leading edge LE and separates into two streams which pass .over the surfaces S1 and S2 in a direction toward the trailing edge TE. On each of the surfaces S1 and S2 of plate 60A boundary layer flow regions are formed immediately adjacent the surfaces S1 and S2. These boundary surface S1 and the line BLl, a second boundary layer flow region is formed in the space between the plate surface S2 and the line-BL2. In FIG. 5a the thicknesses of the boundary layer flow regions have been exaggerated; i.e., the thickness of the first boundary layer flow region is the distance between the surface Sl and the dotted line BLl; the thickness of the second boundary layer region is the distance between the surface S2 and the line BL2. The varying thicknesses of the two boundary layers next to plate A in FIG. 5a are. in fact, much smaller thanillustrated; the boundary layer thicknesses being exaggerated as to their thickness dimension for purposes of more clearly illustrating, inter alia, the attenuation processes hereinafter discussed in detail. A comprehensive treatment of boundary layer flow is contained in the textbook, BOUNDARY- LAYER THEORY by H. Schlicting, translated by J. Kestin, 6th Edition, published by McGraw-I-Iill Book Company.

In the boundary layer flow regions, hereinbefore identified, the fluid flow is rotational in the character. However, the fluid flow is irrotational in character beyond the boundary layer flow region; i.e., above and below the dotted lines BLl and BL2 in FIG. 5a. As indicated in FIGS. '5 and 5a, the fluid flow leaving the trailing edges TE of each of the plates forms oscillatory wakes. These oscillatory wakes include a number of.

vortices. Because of the presence of low frequency acoustic pressure waves, the boundary layer flow regions adjacent the back-to-back surfaces S1 and S2 of each plate include acoustic boundary layers, or layers of oscillating vorticity, wherein vortex flow patterns occur. Thus, because of the presence of the low frequency acoustic pressure waves, the boundary layer flow regions are said to be oscillatory. This is discussed in greater detail hereinafter with reference to FIGS. 6, 7, 8 and 9.

FIGS. 6, 7, 8 and 9' are graphical representations showing fluid flow velocity profiles adjacent a plate sur face S1 at four successive instants of time. These figures also include separate velocity profiles of fluid particle movement caused by the low frequency acoustic pressure waves. For example, in FIG. 6 the surface S1 of the plate 60A is illustrated as extending longitudinally along an X axis, or abscissa, from a point 0 on the surface S1. The upwardly extending Y axis, or ordinate, represents the distance from point 0 on the abscissa from the surface S1 extending into the boundary layer flow region above the surface S1.

In FIG. 6, the curved line W is the envelope of the i mean fluid flow velocities within and beyond the boundary layer flow region above the surface S1 of the plate 60A. The mean fluid flow velocities are represented by the vectors W. Thus, the curved line W is the locus or envelope of the terminations of the individual velocity vectors W. Beginning at theflsurface S1 the fluid flow velocity vector W is zero and increases with increasing distance in a direction parallel to the Y axis from the surface S1. Also illustrated in FIG. 6 is an-' other curved line w which is the locus or envelope of axially-directed acoustic particle velocities w. The

curved line W represents the envelope of locus of the terminations'of the individual acoustic particle velocitiesWat a given instant of time. Hence, individual fluid particles, being'influen'ced by a fluid flow pressure and an acoustic pressure, have a total velocity V (W W). In FIG. 6 the curved line V represents the en lope or locus of the summation of the two velocities W and In FIGS. 7, 8 and 9 envelopes w, W and V are shown together with representative velocity vectors vi, W and l V at other successive instants of time. A comparison of FIGS. 6,7, 8 and 9 indicates that the total velocity V varies from instant to instant and, as a result, the

boundary layer regions become oscillatory. The fluid flow patterns in the boundary layer regions cause an oscillating layer of vorticity to be set up in the boundary layer regions. The layer of oscillatory vorticity thus set up is shed in the form of an oscillatory wake from the trailing edges TE of each of the plates in each series of the attenuator 20. For example, the oscillatory wakes are shed from the trailing edges TE of each of the plates 58A, 58B, 58N. For two successive plates in series,

' the plates in the series attenuation of the low frequency content of the acoustic pressure waves is repeated. As

indicated in FIG. the oscillatory wakes being shed I from the trailing edges'TE of each of the plates eventually decay through viscous dissipation.

FIG. 10 is a graphical representation indicating the ried'out for an attenuator like the attenuator insertion loss :provided by the attenuator 20 in the acoustic propagating flow path shown in FIG. 2. As indicated at FIG. 10, the ordinate represents A; octave sound power levels .in db re l0- watts (i.e., the decibel I lb/sec. mass flow of air, 6.39 lb/sec. mass flow of air, and 9.39 lb/sec. mass flow of air. Explanations of, as

wellas significant features pertaining to, A2 octave sound levels and cen terband frequenciesare discussed in the text HANDBOOK OF NOISE MEASUREMENT (6th Edition), authored by Arnold P. G. Peterson and Ervin E. Gross, 'Jr., published by General Radio Company. Thus, FIG. 10 represents in graphical form actual tests of attenuated sound power level for four different mass flows through which acoustic frequencies as indicated are propagating. The result of incorporating the -attenuator 20 '(FIG. 20) is clearly. evident in that the power levels are significantly diminished and the lower *cehterband frequencies indicating that significant attenuation occurs, especially at the lower frequencies.

Indeed, attenuation can be observed to occur across a broad band of frequencies.

' FIG. 11 is a graphical representation showing calculated insertion loss versus frequency at 4 flow conditions: e.g., where M (the mean flow Mach number) is .0, 0.0274, 0.043, and 0.064. The calculations are car-' wherein the overall length between the leading edge and trailing edge of each plate is L 0.75 inch; 2b 0.33 inch, where b the half width of the channel formed between two plates.

As can be seen from FIG. 11, the insertion loss, especially at the lower frequencies, increases markedly as flow increases from a mean flow Mach number of 0 through 0.064. It is also apparent from FIG. 11 that at the higher frequencies (e.g., circa 10,000 hz), an appreciable attenuation also occurs.

FIG. 12 is a set of curves based on a calculation showing the insertion loss as function of frequency for different numbers N of plates arranged in series in an attenuator, like the attenuator 20. As indicated, the number N of serially arranged plates increased from through 200 to 300 the insertion loss increases at the lower frequencies. It will be noted that in order to provide the various numbers N of plates in series, for a fixed overall length between inlet 30 and outlet 32 of the said device 20, the ratio L/2b is also changed to accommodate the required number N of plates in each series.

FIG. 13 is a view similar to that shown in FIG. 4 of the arrangement of plates in a second embodiment of the invention. As indicated in FIG. 13 some of the plates have a thicker cross-section dimension than others. This is done to enhance intermediate and higher frequencies attenuation.

FIG. 14 is another view similar to that shown in FIG. 4 showing a third embodiment of the invention wherein each of the plates is arranged in a row-and-column away without staggering the various plates, as is the case in FIG. 4. In FIG. 4, an advantage is gained in reducing the overall size of the attenuator 20 by staggering intermediate rows-and-columns of plates.

FIG. 15 is a view similar to that shown in FIG. 14 and FIG. 4 and FIG. 15 is a fourth embodiment of the invention which is in effect a variation of the embodiment shown in FIG. 14. In FIG. 15, some of the plates have a thicker cross-section dimension than others in order to improve intermediate and high frequency attenuation characteristics of the attenuator.

FIG. 16 is a fifth embodiment of the invention wherein, as shown, the plates in successive columns are canted or aligned at an angle so that the incoming fluid flow meets each plate at other than 0 angle of incidence which improves the intermediate and high frequency attenuation Moreover, some of the plates have a' thicker cross-section dimension than others in order to further improve intermediate and high frequency attenuation characteristics. I

While specific embodiments of the invention have been illustrated and described in some detail for the purpose of illustrating the invention, it is'to be understood that the invention may be otherwise embodied without departing from the spirit and scope of the invention which is hereinafter defined in the appended claims.

What is claimed is:

. 1. Apparatus for attenuating the acoustic energy content of acoustic pressure waves propagating through a flowing'fluid comprising: a conduit having inlet and outlet openings adapted for the entrance and exit, re-

spectively, of the flowing fluid; and, at least one series of substantially rigid, acoustically nonabsorpt'ive plates mounted within said conduit and extending from said inlet opening toward said outlet opening, each plate in extending from the leading edge to'the trailing edge of the plate in a direction substantially parallel to the flow of said fluid, said plates in said" seriesibeing spaced apart so that a space exists between thetrailing edge of one plate and the leading edge of the next succeeding plate in the series. a

2. The apparatus according to claim 1 wherein some of said plates have thicker cross sections than "others.

3. Apparatus for attenuating the acoustic energy content of low frequency acoustic pressure wavespropagating through a flow ing fluid comprising: 'aconduit having inlet and outlet openings for the entrance and exit, respectively, of the flowing fluid, and, at least one series of substantially rigid, acoustically nonabsorptive plates mounted within said conduit and extending from said inlet opening toward said outlet opening, each plate in the series having a leading edg'e which is initially contacted by the flowing fluid and a trailing edge from whichthe'flowing fluid leaves the plate, each plate in said series having at least one surface extending from the leading edge to the trailing edge of the plate in a direction substantially parallel to the flow of said fluid, said plates in said series being spaced apart so that a space exists between the trailing edge of one plate and the leading edge of the next succeeding plate in the seriesso that the flowing fluid forms a boundary layer flow region on the surface of the first plate encountered in the series andreeforms newtbo undary layer flow regions on the surfaces of each of the successive plates encountered in the series, the low frequency acoustic pressure waves causing said boundary layer flow regions to become oscillatory whereby each oscillatory boundary layer flow region in succession substantially impedes propagation of low frequency acoustic pressure waves.

4. Apparatus for attenuating the acoustic energy content of acoustic pressure waves, includn'g at least those of low frequency,- propagating through a flowing fluid comprising: an elongated conduit having inlet and outlet openings spaced apart from each other and located at opposite ends of a longitudinally extending central axis of said conduit, said openings being in planes which are parallel with each other,said central axis being orthogonally disposed with respect to said planes, said inlet and outlet openings providing an entrance and exit, respectively, for the flowing fluid and the low frequency acoustic pressure waves propagating through the fluid; and, one row-and-column array of relatively thin acoustically nonabsorptive plates mounted within said conduit, each thin plate in said array having a leading edge which is initially contacted conduit with the'back-to -back parallel surfaces of each plate in the series alori'g saidline' being inparalleliwith ,zsaidjlongitudinafcentral edges ofsuccessiveseiially I 'umn" being separated f by 'longitudinally extending s,; the trailing and leading rranged plates in each coli spaces, each rowof platesin' said array having included therein one plateof each eoluinn" in said array and extending along a line crosswise of' said central axis, the

flowing fluid,'-including the acoustic pressure waves propagating therethrough, passing through said conduit in a direction generally paralleli'with' said central axis from said inlet opening toward said outlet opening, the

flowing' fluidforniing, during its passage through said conduit, separateback-to back boundary layer flow regions on the baek-to-back surfaces, respectively, of

each of the serially-arranged plates in each column of H said array ,'said low' frequency acoustic pressure waves exciting the boundary'l'ayer flow regions and causing said boundary layer flow regions tdbecome'oscillitory thereby forming in each oscillatory boundarylayer flow region a layer of oscillating vorticity wherein flow vortices form, each said layer of vorticity be'ing shed from the trailing edge of each plate in the form of an oscillatory wake into the longitudinally extending space between the trailing edge of one plate and the leading edge of the next succceeding plate in the column whereby propagation of the low frequency acoustic pressure waves through said oscillatory boundary layer flow regions, said layers'of vorticity and said oscillatory by the flowing fluid and a trailing edge from which the posed in back-to-back relationship, each column of plates in said array extending along a line parallel with said central axis, the plates in each said column being arranged in a series along said line and extending from u said inlet opening toward said outlet opening of said others.

wakes is substantially impeded so that the acoustic energy content of said low frequency acoustic, pressure waves is attenuated:

5. The apparatus according to claim 4, wherein some of said plateshaverelatively thicker cross sections than 6. The apparatusaccording to claim5 wherein the plates having the thicker crosssections are included in each row and column of said array and are arranged in symmetrical relationship with respect to said central ans 7. The apparatus according to clairn 4 further comprisng a second row-and-column array of like plates, the rows and columns of saidsecond array of plates being staggered with respect to the rows and columns of said one array. I 1

8. Apparatus for attenuating the acoustic energy content of acoustic pressure waves, including at least low frequency waves, propagating through a flowing fluid comprising: a conduit having inlet and outlet openings for the admission and discharge of the flowing fluid; and, a plurality of plates mounted within said conduit, each plate having a leading edge and a trailing edge and a pair of parallel, relatively flat, acousticallynonabsorptive back-to-back surfaces essentially parallel to the fluid flow extending from the leading edge to the trailing edge, said plates of said plurality being spaced apart from one and other so that'the flowing fluid can establish individual pairs of boundary layer flow regions on said back-to-back surfaces of each platesuccessively encountered as the fluid flows from said inlet opening toward said outlet opening, the low frequency acoustic pressure waves causing each boundary layer flow region to become an oscillatory boundary layer flow region wherein fluid flow vorticities are formed and shed from the trailing edges of each plate.

9. The apparatus according to claim 8 wherein some" 1 plates in said plurality have thicker cross section dimension between the parallel surfaces thereof than other plates in said plurality.

10. Apparatus for attenuating the acoustic energy content of acoustic pressure wave propagating through a flowing fluid comprising? one series of stationary plates mounted in a flow path adapted to be traversed by the flowing fluid and by the acoustic pressure waves propagating through the flowing fluid, said series of plates extending longitudinally in the general direction of the flow of the fluid, each plate in said series having a leading edge and a trailing edge longitudinally spaced therefrom, each plate havinga pair of relatively flat,

acoustically nonabsorptive parallel surfaces disposed in back-to-back relationship and extending longitudinally from said leading edge thereof to said trailing edge,

thereof, in a direction substantially parallel to the flow of said fluid, the distance between said pair of surfaces being relatively smaller than the distance from said leading edge to said trailing edge, the plates in said series being longitudinally separated so that a longitudinally extending space exists between the trailing and leading edges of successive plates in said series.

11. Apparatus according to claim wherein the acoustic pressure waves include at least low frequency acoustic pressure waves and wherein said distance from the leading edge to the trailing edge of each plate as well as said longitudinally extending space between the leading and trailing edges of successive plates are of sufficient length to insure the formation of new pairs of boundary layer flow regions on the surfaces of each plate successively encountered by the flowing fluid.

12. Apparatus according to claim 10 wherein the distance between the pair of surfaces is greater for some plates in the series than for others.

13. Apparatus according to claim 10 further comprising additional series of like plates said additional series together with said one series forming one row-andcolumn array of plates.

14. Apparatus according to claim 13 wherein some plates in said one and additional series have greater distance between the pairs of surfaces thereof than other plates in said one and additional series.

15. Apparatus according to claim 13 wherein said plates having the greater distance between the pairs of surfaces thereof are symmetrically spaced throughout the row-and-column array.

16. Apparatus according to claim 13 further comprising a second row-and-column array of like plates arranged in staggered relationship relative to said one row-and-column array.

17. Apparatus according to claim 16 wherein in each array said longitudinally extending space between the trailing and leading edges of successive plates in the same column is substantially equal to the distance L between the leading and trailing edges of each plate and wherein in each array the distance 2b between the surfaces of successive plates in the same row is substantially twice the distance b between the trailing edge of a plate in one row of one array to the leading edge of a corresponding plate in a corresponding row of the second array whereby the plates in the columns of said second array divide the fluid flowing between adjacent columns in the one array.

. 18. Apparatus according to claim 17 wherein in each column of both arrays there are N plates in series.

19. Apparatus according to claim 18 wherein N ranges from to 300 inclusive and the ratio L/2b ranges from 3 to 1.03, inclusive.

20. Apparatus according to claim 19 wherein some plates in both arrays have more thickness than the remaining plates.

21. Apparatus according to claim 20 wherein said plates are formed from metal corrugations.

22. Apparatus according to claim 21 wherein said plates are formed from strips of corrugated metal adjacently arranged.

23. Apparatus for attenuating the acoustic energy contact of acoustic pressure waves propagating through a flowing fluid comprising: a series of longitudinally spaced acoustically nonabsorptive surfaces mounted in a flow path substantially parallel thereto adapted to be traversed by the flowing fluid, said surfaces in said series having sufficient length and sufficient spacing between successive surfaces to enable the flowing fluid to establish a new boundary layer flow region on each surface successively encountered.

24. The apparatus accoading to claim 23 wherein said surfaces include a leading edge first contacted by the flowing fluid and forming separation bubbles thereat.

25. The apparatus according to claim 23 wherein said acoustic pressure waves are of low frequency and cause said boundary layer flow regions to become oscillatory. 

1. Apparatus for attenuating the acoustic energy content of acoustic pressure waves propagating through a flowing fluid comprising: a conduit having inlet and outlet openings adapted for the entrance and exit, respectively, of the flowing fluid; and, at least one series of substantially rigid, acoustically nonabsorptive plates mounted within said conduit and extending from said inlet opening toward said outlet opening, each plate in said series having a leading edge which is initially contactable by the flowing fluid and a trailing edge from which the flowing fluid can leave the plate, each plate in said series having at least one relatively flat surface extending from the leading edge to the trailing edge of the plate in a direction substantially parallel to the flow of said fluid, said plates in said series being spaced apart so that a space exists between the trailing edge of one plate and the leading edge of the next succeeding plate in the series.
 2. The apparatus according to claim 1 wherein some of said plates have thicker cross sections than others.
 3. Apparatus for attenuating the acoustic energy content of low frequency acoustic pressure waves propagating through a flowing fluid comprising: a conduit having inlet and outlet openings for the entrance and exit, respectively, of the flowing fluid, and, at least one series of substantially rigid, acoustically nonabsorptive plates mounted within said conduit and extending from said inlet opening toward said outlet opening, each plate in the series having a leading edge which is initially contacted by the flowing fluid and a trailing edge from which the flowing fluid leaves the plate, each plate in said series having at lEast one surface extending from the leading edge to the trailing edge of the plate in a direction substantially parallel to the flow of said fluid, said plates in said series being spaced apart so that a space exists between the trailing edge of one plate and the leading edge of the next succeeding plate in the series so that the flowing fluid forms a boundary layer flow region on the surface of the first plate encountered in the series and re-forms new boundary layer flow regions on the surfaces of each of the successive plates encountered in the series, the low frequency acoustic pressure waves causing said boundary layer flow regions to become oscillatory whereby each oscillatory boundary layer flow region in succession substantially impedes propagation of low frequency acoustic pressure waves.
 4. Apparatus for attenuating the acoustic energy content of acoustic pressure waves, includng at least those of low frequency, propagating through a flowing fluid comprising: an elongated conduit having inlet and outlet openings spaced apart from each other and located at opposite ends of a longitudinally extending central axis of said conduit, said openings being in planes which are parallel with each other, said central axis being orthogonally disposed with respect to said planes, said inlet and outlet openings providing an entrance and exit, respectively, for the flowing fluid and the low frequency acoustic pressure waves propagating through the fluid; and, one row-and-column array of relatively thin acoustically nonabsorptive plates mounted within said conduit, each thin plate in said array having a leading edge which is initially contacted by the flowing fluid and a trailing edge from which the flowing fluid subsequently leaves the plate, each thin plate having a pair of relatively flat, parallel surfaces which extend longitudinally from its leading edge to its trailing edge, said pair of parallel surfaces being disposed in back-to-back relationship, each column of plates in said array extending along a line parallel with said central axis, the plates in each said column being arranged in a series along said line and extending from said inlet opening toward said outlet opening of said conduit with the back-to-back parallel surfaces of each plate in the series along said line being in parallel with said longitudinal central axis, the trailing and leading edges of successive serially-arranged plates in each column being separated by longitudinally extending spaces, each row of plates in said array having included therein one plate of each column in said array and extending along a line crosswise of said central axis, the flowing fluid, including the acoustic pressure waves propagating therethrough, passing through said conduit in a direction generally parallel with said central axis from said inlet opening toward said outlet opening, the flowing fluid forming, during its passage through said conduit, separate back-to-back boundary layer flow regions on the back-to-back surfaces, respectively, of each of the serially-arranged plates in each column of said array, said low frequency acoustic pressure waves exciting the boundary layer flow regions and causing said boundary layer flow regions to become oscillatory thereby forming in each oscillatory boundary layer flow region a layer of oscillating vorticity wherein flow vortices form, each said layer of vorticity being shed from the trailing edge of each plate in the form of an oscillatory wake into the longitudinally extending space between the trailing edge of one plate and the leading edge of the next succceeding plate in the column whereby propagation of the low frequency acoustic pressure waves through said oscillatory boundary layer flow regions, said layers of vorticity and said oscillatory wakes is substantially impeded so that the acoustic energy content of said low frequency acoustic pressure waves is attenuated.
 5. The apparatus according to claim 4 wherein some of said plates have relatively thicker cross sections Than others.
 6. The apparatus according to claim 5 wherein the plates having the thicker cross sections are included in each row and column of said array and are arranged in symmetrical relationship with respect to said central axis.
 7. The apparatus according to claim 4 further comprisng a second row-and-column array of like plates, the rows and columns of said second array of plates being staggered with respect to the rows and columns of said one array.
 8. Apparatus for attenuating the acoustic energy content of acoustic pressure waves, including at least low frequency waves, propagating through a flowing fluid comprising: a conduit having inlet and outlet openings for the admission and discharge of the flowing fluid; and, a plurality of plates mounted within said conduit, each plate having a leading edge and a trailing edge and a pair of parallel, relatively flat, acoustically nonabsorptive back-to-back surfaces essentially parallel to the fluid flow extending from the leading edge to the trailing edge, said plates of said plurality being spaced apart from one and other so that the flowing fluid can establish individual pairs of boundary layer flow regions on said back-to-back surfaces of each plate successively encountered as the fluid flows from said inlet opening toward said outlet opening, the low frequency acoustic pressure waves causing each boundary layer flow region to become an oscillatory boundary layer flow region wherein fluid flow vorticities are formed and shed from the trailing edges of each plate.
 9. The apparatus according to claim 8 wherein some plates in said plurality have thicker cross section dimension between the parallel surfaces thereof than other plates in said plurality.
 10. Apparatus for attenuating the acoustic energy content of acoustic pressure wave propagating through a flowing fluid comprising: one series of stationary plates mounted in a flow path adapted to be traversed by the flowing fluid and by the acoustic pressure waves propagating through the flowing fluid, said series of plates extending longitudinally in the general direction of the flow of the fluid, each plate in said series having a leading edge and a trailing edge longitudinally spaced therefrom, each plate having a pair of relatively flat, acoustically nonabsorptive parallel surfaces disposed in back-to-back relationship and extending longitudinally from said leading edge thereof to said trailing edge thereof, in a direction substantially parallel to the flow of said fluid, the distance between said pair of surfaces being relatively smaller than the distance from said leading edge to said trailing edge, the plates in said series being longitudinally separated so that a longitudinally extending space exists between the trailing and leading edges of successive plates in said series.
 11. Apparatus according to claim 10 wherein the acoustic pressure waves include at least low frequency acoustic pressure waves and wherein said distance from the leading edge to the trailing edge of each plate as well as said longitudinally extending space between the leading and trailing edges of successive plates are of sufficient length to insure the formation of new pairs of boundary layer flow regions on the surfaces of each plate successively encountered by the flowing fluid.
 12. Apparatus according to claim 10 wherein the distance between the pair of surfaces is greater for some plates in the series than for others.
 13. Apparatus according to claim 10 further comprising additional series of like plates said additional series together with said one series forming one row-and-column array of plates.
 14. Apparatus according to claim 13 wherein some plates in said one and additional series have greater distance between the pairs of surfaces thereof than other plates in said one and additional series.
 15. Apparatus according to claim 13 wherein said plates having the greater distance between the pairs of surfaces thereof are symmetrically spaced throughout the row-anD-column array.
 16. Apparatus according to claim 13 further comprising a second row-and-column array of like plates arranged in staggered relationship relative to said one row-and-column array.
 17. Apparatus according to claim 16 wherein in each array said longitudinally extending space between the trailing and leading edges of successive plates in the same column is substantially equal to the distance L between the leading and trailing edges of each plate and wherein in each array the distance 2b between the surfaces of successive plates in the same row is substantially twice the distance b between the trailing edge of a plate in one row of one array to the leading edge of a corresponding plate in a corresponding row of the second array whereby the plates in the columns of said second array divide the fluid flowing between adjacent columns in the one array.
 18. Apparatus according to claim 17 wherein in each column of both arrays there are N plates in series.
 19. Apparatus according to claim 18 wherein N ranges from 100 to 300 inclusive and the ratio L/2b ranges from 3 to 1.03, inclusive.
 20. Apparatus according to claim 19 wherein some plates in both arrays have more thickness than the remaining plates.
 21. Apparatus according to claim 20 wherein said plates are formed from metal corrugations.
 22. Apparatus according to claim 21 wherein said plates are formed from strips of corrugated metal adjacently arranged.
 23. Apparatus for attenuating the acoustic energy contact of acoustic pressure waves propagating through a flowing fluid comprising: a series of longitudinally spaced acoustically nonabsorptive surfaces mounted in a flow path substantially parallel thereto adapted to be traversed by the flowing fluid, said surfaces in said series having sufficient length and sufficient spacing between successive surfaces to enable the flowing fluid to establish a new boundary layer flow region on each surface successively encountered.
 24. The apparatus accoading to claim 23 wherein said surfaces include a leading edge first contacted by the flowing fluid and forming separation bubbles thereat.
 25. The apparatus according to claim 23 wherein said acoustic pressure waves are of low frequency and cause said boundary layer flow regions to become oscillatory. 